B&O's CPL signals have been a distinctive feature of all lines
radiating from Cumberland, MD, and indeed most B&O main lines everywhere.

The first permanent installation for B&O CPL's was on the Staten Island
Rapid Transit, when it was electrified beginning in the summer of 1924.
This gave the B&O an opportunity to operate the new
signal system in a heavy traffic situation. SIRT had the highest density
passenger train traffic of any B&O or B&O subsidiary divison.

The second installment was along the 12 mile Perth Amboy Subdivision
between Clifton Jct and Tottenville as electrification progressed. The
Tottenville line began full electric service July 1, 1925. The SIRT North Shore
line from St. George to Arlington was electrified by late 1925. After these
prototype installations, the CPLs became the system standard.

Even though CSX is replacing these signals with the modern "traffic light" style
installations, there are still numerous CPLs left on the B&O parts of the system. The Keystone
and Cumberland Terminal Subdivisions' conversions are complete as of 12/2002, but both the Mountain
and Cumberland Subs still have them in many places, including a few on bridges and cantilevers.

Signal Aspects & IndicationsAccording to the 1941 B&O Rules and Regulations of the Operating Department.

The lunars/yellows have two indications:position vertically and position
horizontally.

Vertically: Above the main head it's track speed from here, below it's
medium and none is slow.

Horizontally: Next signal is track speed if in the middle, medium if on
the left and slow on the right.

This series of pictures taken at Hansrote on the Cumberland Sub illustrates
a train passing a CPL. The proceed indication goes off, and after a short delay the permissive
stop indication comes on. Depending on the circuitry, a restrictive indication might be displayed
while the relays complete their cycles.

Q134 eastbound passing the Hansrote CPL on the Cumberland Subdivision, 5/13/2001.(click for a larger image)

Suggested links for further reading on the topic of CPLs and signalling in general: